Several aspects of the interaction of the complement system with Actinomyces viscosus T14V, an organism implicated in the pathogenesis of periodontal disease, are under exploration. Complement fixation by monoclonal antibodies is being used to localize epitopes on the fimbriae of these bacteria and associate them with functional properties of the fimbriae. Antibodies with different specificities for the type 1 fimbriae (which mediate attachment to saliva treated hydroxyapatite) and the type 2 fimbriae (which are associated with lectin activity for saccharide receptors on other bacteria and mammalian cells) have been found to cooperate in activation of complement. Thus, pairs of antibodies, at concentrations of each which fail to activate complement, initiate the activation of this mediator system. The epitopes with which the antibodies in each of these pairs react must be within a spatial distance which permits bridging by a Clq molecule and subsequent activation of the complement sequence. The repeating epitope sequence of the type 2 fimbriae apparently differs considerably from that of the type 1 fimbriae since high concentrations of antibodies specific for the type 1 fimbriae activate complement whereas the majority of those specific for the type 2 fimbriae do not. The participation of both antibodies in the synergistic activation of complement was demonstrated by the finding that the combination of an F(ab')2 fragment of one antibody with an intact antibody specific for a different epitope was ineffective. Also, binding of one antibody failed to significantly enhance the binding of the second antibody, a condition which could result in increased activation of the complement system.